Finite quantum dissipation: the challenge of obtaining specific heat

P. Hänggi, G.-L. Ingold, and P. Talkner

New J. Phys. 10, 115008 (2008) DOI: 10.1088/1367-2630/10/11/115008

We consider a free particle coupled with finite strength to a bath and investigate the evaluation of its specific heat. A harmonic oscillator bath of Drude type with cutoff frequency ωD is employed to model an ohmic friction force with dissipation strength γ. Two scenarios for obtaining specific heat are presented. The first one uses the measurement of the kinetic energy of the free particle while the second one is based on the reduced partition function. Both descriptions yield results which are consistent with the Third Law of thermodynamics. Nevertheless, the two methods produce different results that disagree even in their leading quantum corrections at high temperatures. We also consider the regime where the cutoff frequency is smaller than the friction strength, i.e. ωD<γ. There, we encounter puzzling results at low temperatures where the specific heat based on the thermodynamic prescription becomes negative. This anomaly is rooted in an ill-defined density of states of the damped free particle which assumes unphysical negative values when γ/ωD>1.